A known tandem image forming apparatus is provided with image carriers in correspondence with yellow, magenta, cyan and black toner. Such a tandem image forming apparatus can print in color at substantially the same speed as printing in monochrome, because a toner image of each color is formed at substantially the same time on each corresponding one of the image carriers and each of the different colored images is sequentially laid on top of a sheet while the sheet passes through the image carriers.
As disclosed in U.S. Pat. No. 6,708,011, a tandem image forming apparatus includes an image carrier cartridge for integrally supporting image carriers, each associated with one color. The image carrier cartridge is slidably installed in or removed from a housing of the image forming apparatus. A developing cartridge for developing an electrostatic latent image formed on each of the image carriers is removably set in the image carrier cartridge.
The developing cartridge includes a developing roller for supplying toner to the image carrier. To properly transfer the toner from the developing roller to the image carrier, the developing roller has to be pressed against the image carrier at an appropriate pressure. To properly press the developing roller against the image carrier, an elastic member, such as a spring, is provided in the housing of the image forming apparatus. The developing roller is urged by the elastic member to press the developing roller against the image carrier.
However, elasticity of the elastic member will gradually decrease as the elastic member deteriorates. Accordingly, the pressing force of the developing roller against the image carrier decreases, leading to toner supply shortage. The toner supply shortage may cause development failure.